Identify the condition. Hypothermia can strike suddenly, but usually there is a range of progressive symptoms that become evident as the victim succumbs. These can start with shivering, clumsiness, and slurred speech, progressing into confusion and apathy, drowsiness, and then shallow breathing and a low pulse rate.
It's not uncommon for the victim to be unaware that he or she is suffering from hypothermia because of the gradual onset of symptoms. Be sure that all members of your party are aware of the symptoms and check each other during weather conditions conducive to hypothermia.